Thermal and electrical behaviour of a resistive joint in the ATLAS toroids

ATLAS air core toroids exploit aluminum-clad NbTi/Cu superconducting cables. Several joints along the conductor are presently foreseen, e.g, between pancakes and between coils. The operating current (20.5 kA) is about 4 times larger than in the previous comparable magnets, and so the power dissipated in a similar joint is substantially higher, making the resistance value critical. In this work we have developed some analytical models describing the temperature profile along the conductor given a localized or distributed heat source. By means of these models we found the highest resistance permissible in order not to exceed a given local temperature rise, Later we have calculated through a 2-D finite elements program the specific resistance that can be expected when the joint is performed by TIG-welding the aluminum matrices. Such a value depends strongly on the aluminum-copper interface resistance and on the aluminum RRR in the welding region. We have shown that with reasonable assumptions specific resistances lower than 10(-9) Ohm m should be achieved, The results from the first measurements, confirming such a value, are reported and discussed.